In recent years, various proposals have been made with regard to optical scanning optical systems (and optical scanning devices) for scanningly deflecting light wherein an optical deflector is constituted by using an oscillator device configured to provide sinusoidal vibration based on the resonance phenomenon.
The optical scanning optical systems with such resonance type optical deflector have the following features, as compared with an optical scanning optical system using a rotary polygonal mirror such as a polygon mirror.
Namely, the size of the optical deflecting device can be made quite small, and the electric power consumption can be made very slow.
On the other hand, in the resonance type deflector mentioned above, since the deflection angle (displacement angle) of the mirror changes theoretically sinusoidally, the angular speed is not constant.
Conventionally, in an attempt to correcting the characteristic of the resonance type deflector such as described above, U.S. Pat. No. 5,047,630 and U.S. Pat. No. 7,271,943 have proposed an optical deflector as follows.
In U.S. Pat. No. 5,047,630, a resonance type deflector having oscillation modes of a fundamental frequency and a frequency three-fold the fundamental frequency is used to realize the chopping wave driving.
FIG. 8 illustrates a micromirror which realizes approximately chopping-wave-like driving in the U.S. Pat. No. 5,047,630.
As shown in FIG. 8, an optical deflecting device 12 here comprises oscillators 14 and 16, torsion springs 18 and 20, driving members 23 and 50, detecting elements 15 and 32, and a control circuit 30.
This micromirror has a fundamental resonance frequency and a resonance frequency of approximately three-fold the same, and it drives with a resultant frequency of the fundamental frequency and the triple frequency.
With this arrangement, the oscillator 14 having a mirror surface is driven by chopping wave driving, and optical deflection can be done with a deflection angle in which the change of the angular speed is fewer than the sinusoidal driving.
In operation, the oscillation of the oscillator 14 is detected by the detecting elements 15 and 32, and a necessary driving signal for the chopping wave driving is generated by the control circuit 30. Through the driving members 23 and 50, the micromirror is driven.
On the other hand, U.S. Pat. No. 7,271,943 discloses a micro-oscillator in which a system including a plurality of torsion springs and a plurality of movable elements has a plurality of discrete natural oscillation modes.
In this micro-oscillator, in the plurality of discrete natural oscillation modes, a reference oscillation mode which is a natural oscillation mode of a reference frequency and an even-multiple oscillation mode which is a there is the even multiple that is the of natural oscillation mode of a frequency approximately an N-fold the reference frequency (N is an even number) are included.
In U.S. Pat. No. 7,271,943, the micro-oscillator is oscillated in these oscillation modes, whereby sawtooth wave driving is realized.
On the other hand, in resonance type deflectors, the resonance frequency of the oscillator device would have dispersion due to manufacturing errors.
For lower power consumption, driving around the resonance frequency is desirable. Therefore, tuning of the resonance frequency is necessary.
Furthermore, in an image forming apparatus using an optical deflector comprised of such actuator, for stabilized aspect ratio of the image and lower deterioration of the picture quality, there is a necessity of tuning the resonance frequency of the optical deflector to a predetermined value.
Conventionally, for adjustment of the resonance frequency such as mentioned above, Japanese Laid-Open Patent Application No. 2002-40355 and Japanese Laid-Open Patent Application No. 2004-219889 have proposed a planar type galvano mirror or an oscillation mirror as follows.
Japanese Laid-Open Patent Application No. 2002-40355 uses, as shown in FIG. 9, a planar type galvano mirror having mass loading members 1001 and 1002 formed at the opposite ends of a movable plate which is comprised of an electric coil and a reflection surface resiliently supported around a torsion axis for oscillation.
The mass is removed by irradiating a laser beam to the mass loading members 1001 and 1002 of this galvano mirror or, alternatively, a resin is applied to enlarge the mass, whereby the inertia moment is adjusted to tune the frequency at a predetermined value.
Furthermore, Japanese Laid-Open Patent Application No. 2004-219889 uses a method in which the resonance frequency of the oscillator device is set slightly high beforehand and, by adding a mass to a portion of the oscillator, the resonance frequency is tuned to a desired level.